“Missing Link Clusters” is a Chandra Large Project to examine rich open stellar clusters aged 5-30 Myr that form a “missing link” in our understanding of evolution of pre-main-sequence (PMS) stellar activity. Chandra data for half of 12 proposed missing link clusters within 2 kpc distance have already been analyzed. Here, we report on the characterization of the cluster membership, distances and ages, stellar X-ray and bolometric luminosities, and stellar masses employing Gaia, 2MASS, Spitzer, WISE and various ground-based optical-infrared photometric data. The goal is to derive mass-stratified activity-age-rotation relations in early stellar evolution for the combined sample of ~6,000 “missing link” stars and >30,000 younger (<5 Myr) stars from our published MYStIX and SFiNCs projects. Our preliminary results show that PMS X-ray luminosities systematically decrease with time for >0.5 M⊙ stars and X-ray luminosity turnover points shift. Both effects may be due to the combination of stellar contractions on Hayashi tracks and transitions from Hayashi to Henyey tracks.